The present invention relates to a method of mounting an electronic part with bumps projectingly provided from electrodes thereof on electrodes of a circuit board in which the bumps are brought into contact with the electrodes of the circuit board and thereafter a reflow soldering is effected to heat and melt the bumps, thereby the bumps are bonded onto the electrodes of the circuit board.
In recent years, electronic parts with bumps have been spread because electronic parts with bumps occupy a small area in comparison with electronic parts with leads. FIG. 14 is a side view of an electronic part with bumps mounted on the board in the conventional manner. The electronic part with bumps 1 is formed by projectingly providing bumps (projecting electrodes) 4 from electrodes 3 of an electronic part 2 by means of a solder. The bumps 4 are brought into contact with electrodes 6 of a circuit board 5, and thereafter a reflow soldering is effected to heat and melt the bumps 4, thereby bonding the bumps 4 to the electrodes 6.
FIG. 15 is an enlarged view showing a state before the bump 4 is bonded to the electrode 6. The cross-section of the bump 4 is substantially in a spherical shape. FIG. 16 is an enlarged view showing a state after the bonding. The bump 4 assumes a slightly flat cross-sectional shape by being squeezed due to the surface tension at the time of heating and melting and the dead load of the electronic part 2. However, an area of the electrode 6 of the circuit board 5 is substantially identical with that of the electrode 3 of the electronic part 2, so that the diameter of the bump 4 after reflow soldering is almost the same as that of the bump 4 before reflow soldering.
Although the number of bumps 4 projectingly provided from the electronic part 2 is generally more than ten or more than several hundreds and a great many bumps are provided, all of the bumps 4 must be surely brought into contact with and bonded on the electrodes 6 of the circuit board 5. Even when there is only one bump of which bonding is malfunctional, the electronic part is made to be inferior.
However, since it is unavoidable in manufacture that variation in height d1 of the bumps 4 is resulted and it is unavoidable in manufacture that curvature is resulted in the circuit board 5, in the conventional mounting method, there is a problem that it is difficult to surely bring a large number of the bumps 4 all into contact with the electrodes 6 of the circuit board 5. In this connection, the bumps 4 are formed by a screen printing method or an electro-plating method.
Also, after the electronic part 4 with bumps is mounted on the circuit board 5, it is necessary to inspect whether the bumps are firmly bonded upon the electrodes 6 and the circuit board 5. Since, however, in the conventional mounting method, there is little difference in shape between well bonded and badly bonded bumps 4, there is a problem that even when the appearance inspection is effected, for example, by means of a soft X-ray, it is difficult to obtain a quality inspection result with high reliability.
Incidentally, as a difference .DELTA.=d1-d2 between the height d1 of the bump 4 before the reflow soldering as shown in FIG. 15 and the height d2 of the bump 4 after the reflow soldering as shown in FIG. 16 becomes greater, namely as the height of the bump 4 is more reduced by the reflow soldering, the dispersion of the height of the bump 4 and the curvature of the board 5 can be effectively absorbed and all the bumps can be bonded to the electrodes of the circuit board with certainty. Further, as the difference between the configuration of the bump 4 as shown in FIG. 15 and that of the bump 4 as shown in FIG. 16 becomes greater, it is easier to discriminate the bonding quality by the appearance inspection.
Accordingly, the present invention has been made in view of the foregoing points and has its object to provide a method of mounting an electronic part with bumps on a circuit board, in which all the bumps are surely bonded to the electrodes of the circuit board and which can accurately determine the bonding quality in the appearance inspection.